The invention refers to a datum sensor for measuring of elongations in buildings of concrete, comprising a carrier member of metal, particularly steel, and designed as a hollow section and carrying the strain gauges and having its end portions, which are, if desired, profilated, designed as adhering portions. The carrier member has, in its middle portion forming the measuring section, the strain gauges fastened at the outer periphery of the hollow section. It is already known to use tubular deformation bodies as carriers for strain gauges, but such bodies have not been embedded within a building and instead have been arranged at the outer side in an analogous manner as were arranged pressure cells.
In connection with the use of tubular carriers for datum sensors there existed furthermore the prejudice that the buckling strength of the carrier will be reduced. With the known datum sensors proposed for measuring of elongations in concrete, which sensors usually comprise a solid and rod-like carrier, a first difficulty is generally encountered on the basis of not taking into consideration the shrinkage behaviour and the micro cracks resulting from such shrinkage. A further difficulty arises when measuring elongations in concrete buildings results from each datum sensor influencing the strength properties within that area within which the measurement is to be effected and thus, from measuring only correspondingly distorted values. It is already known to apply strain gauges on carriers of plastic material. Such carriers consisting of platstic material are different from the concrete surrounding the measuring area to such a great extent that the measured values are only insufficiently correlated to the actual conditions. When measuring elongations in concrete buildings there must additionally be considered that on account of the grain size of the aggregates of the concrete, relatively long carriers are required for the strain gauges to obtain an integrating measurement of greater reliability. However, it is just with such relatively long carriers that, with the known embodiments, the strength properties are strongly distorted within the area of measurement.